A particle beam or ebeam writer uses one or more beams of particles (typically electrons, but other particles may be used) to generate a given pattern on a plate. The plate is covered with a particle sensitive material. By way of example, consider the case of an electron beam writing (EBW) technology. The EBW technology uses an electron beam to generate various patterns on a surface. A surface may be a reticle, a photomask, a stencil mask, a wafer, a fabric, a glass, a plastic, an LCD panel or any other surface.
One use of EBW technology is in writing a reticle or a photomask in optical lithography. Light shines through the reticle in a stepper, a wafer writing machine, to write a substrate such as a silicon wafer, or a fabric, a glass, a plastic, an LCD panel or any other substrate. A fundamental problem with optical lithography is the image quality degradation and the resolution limits caused by optical proximity effect. One method to overcome this problem is using electron beam (ebeam) direct writing (EBDW) technology, a variation of the EBW technology. In EBDW technology, EBDW technology is used to write a substrate directly in lieu of the stepper. The theoretical resolution of an electron beam is finer, which allows writing denser layouts than with optical lithography. However, this technology has a lower throughput.
Several methods have been conventionally used to increase the throughput of using EBW. One such method is based on a variable shape beam (VSB) technology, which facilitates writing patterns by using particle beam shots of fixed and simple shapes with variable size. Generally, an electron beam is shone through a shaping aperture (usually square). The beam exiting the shaping aperture is deflected by a deflector through one of a number of simple shape stencils. A demagnifying lens then reduces the shaped beam onto the target wafer. Using a combination of the simple stencil shapes or one or more portions of the simple stencil shapes, the desired patterns are written to the surface. By way of example, the simple stencil shapes include rectangles and triangles. Further, the VSB-type EBW performs proximity effect correction by dose control, shape biasing and minute fracturing. However, such manipulations increase writing time. Variable shape beam writing is well known in the art.
Another conventional method used for IC fabrication is cell projection (CP) technology, which is also referred to as character projection or block exposure. Like VSB writing, character projection technology directs an ebeam through a first shaping aperture and deflects the first shaped beam to a stencil. Another character projection technology may deflect an ebeam to a first shaping aperture and direct the first shaped beam to a stencil. Yet another character projection technology may use other than two apertures. In any case at least one of the apertures would contain a character of complex shapes in character projection, thereby enabling writing complicated patterns by one exposure shot. As a result, the overall exposure time is decreased. In addition, the writing system throughput increases. However, the technique is limited by several restrictions pertaining to the geometric sizes and kind of figures that can be exposed. In addition, the proximity effect correction becomes a very challenging task. The Coulomb effect also introduces difficulties in the use of charged particle beam writers by blurring the image written by the particle beam writer, thus reducing the accuracy of the writing.
Coulomb's Law tells us that oppositely charged particles will be attracted to each other and that like charged particles will repel each other. In the case of an EBW, the negatively charged electrons repel each other. By the time the electron beam reaches the writing surface the electrons will be more dispersed than when they started, thus creating a “blurred” image. This effect is called the Coulomb effect. The amount of the blurring, in size δ, is given by the formula
                    δ        ∝                  I                      V                          3              2                                                          (        1        )            where I is the beam current and V is the acceleration voltage. A similar effect occurs with positively charged particles.
In order to write finely detailed patterns with an ebeam writer, it is necessary to reduce the amount of blur caused by the Coulomb effect. Considering the above equation, in order to minimize the blur size δ, one needs to reduce the current I, and/or increase the voltage V. However, reducing current and/or increasing voltage cause other difficulties. Higher voltage results in increased back-scattering, which reduces the fidelity of the written design image. Reducing current is undesirable as it increases the exposure time which means the design will take longer to write using the electron beam. Thus we would like to find a way to design cells that reduces blurring, without also increasing backscattering or increasing exposure time.
With cell projection technology, the beam current is proportional to the open aperture dimensions of the cell. As discussed above, the lower the beam current, the less blurring occurs as a result of the Coulomb effect. At the same time, lowering the current will increase the required exposure time.
In order to write the image in the surface of the resist, a certain amount of energy must be transferred from the electrons into the material of the resist layer. The amount of energy transferred is the dose amount. A design must have higher dose amount than a threshold value determined by the resist so that the image is successfully written on the resist, but not so much that too much is written outside of the design area because of scattering effects such as forward scattering in the resist, and backscattering from the material under the resist. The design must also take into consideration the amount of energy deposited in neighboring patterns which could spill over into the design area due to these scattering effects. Adjusting for the amount of charge in order to achieve the correct dose is called dose correction.
Though other systems and methods for reducing the amount of electrons have been proposed, such as attaching mesh structures to the stencil mask, such systems introduce an extra cost in the manufacturing step. Furthermore, mesh structures may overheat and therefore may not be practical. To the inventors' knowledge a production EBW machine that has utilized such a system has not been made.
In light of the foregoing discussion, a need exists for a method and system that improves the throughput of EBW technology and simultaneously maintains high accuracy using the CP system. Thus, a design technology for making characters that reduce maximally Coulomb and proximity effects while maintaining thermal, structural, line edge roughness, and other design considerations within acceptable limits is desired. Such optimization of the balance of different effects need to be differently applied for each character or within different parts of the same character to maximize the reduction of the effects of Coulomb and proximity effects. The present invention addresses such a need.